1. Field of the Invention
Embodiments of the present invention are directed in general to novel phosphors for use in a white light illumination system such as a white light emitting diodes (LED). In particular, the white LED's of the present invention comprise a radiation source emitting in the non-visible to near ultraviolet (UV) to purple wavelength range, a first luminescent material comprising a blue phosphor, and a second luminescent material comprising a yellow phosphor.
2. State of the Art
It has been suggested that white light illumination sources based wholly or in part on the light emitting diode will likely replace the conventional, incandescent light bulb. Such devices are often referred to as “white LED's,” although this may be somewhat of a misnomer, as an LED is generally the component of the system that provides the energy to another component, a phosphor, which emits light of more-or-less one color; the light from several of these phosphors, possibly in addition to the light from the initial pumping LED are mixed to make the white light.
Nonetheless, white LED's are known in the art, and they are relatively recent innovations. It was not until LED's emitting in the blue/ultraviolet region of the electromagnetic spectrum were developed that it became possible to fabricate a white light illumination sources based on an LED. Economically, white LED's have the potential to replace incandescent light sources (light bulbs), particularly as production costs fall and the technology develops further. In particular, the potential of a white light LED is believed to be superior to that of an incandescent bulbs in lifetime, robustness, and efficiency. For example, white light illumination sources based on LED's are expected to meet industry standards for operation lifetimes of 100,000 hours, and efficiencies of 80 to 90 percent. High brightness LED's have already made a substantial impact on such areas of society as traffic light signals, replacing incandescent bulbs, and so it is not surprising that they will soon provide generalized lighting requirements in homes and businesses, as well as other everyday applications.
Chromaticity Coordinates on a CIE Diagram, and the CRI
White light illumination is constructed by mixing various or several monochromatic colors from the visible portion of the electromagnetic spectrum, the visible portion of the spectrum comprising roughly 400 to 700 nm. The human eye is most sensitive to a region between about 475 and 650 nm. To create white light from either a system of LED's, or a system of phosphors pumped by a short wavelength LED, it is necessary to mix light from at least two complementary sources in the proper intensity ratio. The results of the color mixing are commonly displayed in a CIE “chromaticity diagram,” where monochromatic colors are located on the periphery of the diagram, and white at the center. Thus, the objective is to blend colors such that the resulting light may be mapped to coordinates at the center of the diagram.
Another term of art is “color temperature,” which is used to describe the spectral properties of white light illumination. The term does not have any physical meaning for “white light” LED's, but it is used in the art to relate the color coordinates of the white light to the color coordinates achieved by a black-body source. High color temperature LED's versus low color temperature LED's are shown at www.korry.com.
Chromaticity (color coordinates on a CIE chromaticity diagram) has been described by Srivastava et al. in U.S. Pat. No. 6,621,211. The chromaticity of the prior art blue LED-YAG:Ce phosphor white light illumination system described above are located adjacent to the so-called “black body locus,” or BBL, between the temperatures of 6000 and 8000 K. White light illumination systems that display chromaticity coordinates adjacent to the BBL obey Planck's equation (described at column 1, lines 60-65 of that patent), and are desirable because such systems yield white light which is pleasing to a human observer.
The color rendering index (CRI) is a relative measurement of how an illumination system compares to that of a black body radiator. The CRI is equal to 100 if the color coordinates of a set of test colors being illuminated by the white light illumination system are the same as the coordinates generated by the same set of test colors being irradiated by a black body radiator.
Prior Art Approaches to Fabricating White LED's
In general, there have been three general approaches to making white LED's. One is to combine the output from two or more LED semiconductor junctions, such as that emitted from a blue and a yellow LED, or more commonly from a red, green, and blue (RGB) LED's. The second approach is called phosphor conversion, wherein a blue emitting LED semiconductor junction is combined with a phosphor. In the latter situation, some of the photons are down-converted by the phosphor to produce a broad emission centered on a yellow frequency; the yellow color then mixes with other blue photons from the blue emitting LED to create the white light.
Phosphors are widely known, and may be found in such diverse applications as CRT displays, UV lamps, and flat panel displays. Phosphors function by absorbing energy of some form (which may be in the form of a beam of electrons or photons, or electrical current), and then emitting the energy as light in a longer wavelength region in a process known as luminescence. To achieve the required amount of luminescence (brightness) emitted from a white LED, a high intensity semiconductor junction is needed to sufficiently excite the phosphor such that it emits the desired color that will be mixed with other emitted colors to form a light beam that is perceived as white light by the human eye.
In many areas of technology, phosphors are zinc sulfides or yttrium oxides doped with transition metals such as Ag, Mn, Zn, or rare earth metals such as Ce, Eu, or Tb. The transition metals and/or rare earth element dopants in the crystal function as point defects, providing intermediate energy states in the material's bandgap for electrons to occupy as they transit to and from states in the valence band or conduction band. The mechanism for this type of luminescence is related to a temperature dependent fluctuation of the atoms in the crystal lattice, where oscillations of the lattice (phonons) cause displaced electron to escape from the potential traps created by the imperfections. As they relax to initial state energy states they may emit light in the process.
U.S. Pat. No. 5,998,925 to Shimizu et al. discloses the use of a 450 nm blue LED to excite a yellow phosphor comprising a yttrium-aluminum-garnet (YAG) fluorescent material. In this approach a InGaN chip functions as a visible, blue-light emitting LED, and a cerium doped yttrium aluminum garnet (referred to as “YAG:Ce”) serves as a single phosphor in the system. The phosphor typically has the following stoichiometric formula: Y3Al5O12:Ce3+. The blue light emitted by the blue LED excites the phosphor, causing it to emit yellow light, but not all the blue light emitted by the blue LED is absorbed by the phosphor; a portion is transmitted through the phosphor, which then mixes with the yellow light emitted by the phosphor to provide radiation that is perceived by the viewer as white light.
U.S. Pat. No. 6,504,179 to Ellens et al. disclose a white LED based on mixing blue-yellow-green (BYG) colors. The yellow emitting phosphor is a Ce-activated garnet of the rare earths Y, Tb, Gd, Lu, and/or La, where a combination of Y and Tb was preferred. In one embodiment the yellow phosphor was a terbium-aluminum garnet (TbAG) doped with cerium (Tb3Al5O12—Ce). The green emitting phosphor comprised a CaMg chlorosilicate framework doped with Eu (CSEu), and possibly including quantities of further dopants such as Mn. Alternative green phosphors were SrAl2O4:Eu2+ and Sr4Al14O25:Eu2+. New material in replace 5998925, using 450 nm Blue LED to excite mixture of green and yellow phosphors (Tb3Al5O12—Ce).
U.S. Pat. No. 6,649,946 to Bogner et al. disclosed yellow to red phosphors based on alkaline earth silicon nitride materials as host lattices, where the phosphors may be excited by a blue LED emitting at 450 nm. The red to yellow emitting phosphors uses a host lattice of the nitridosilicate type MxSiyNz:Eu, wherein M is at least one of an alkaline earth metal chosen from the group Ca, Sr, and Ba, and wherein z=⅔x+ 4/3y. One example of a material composition is Sr2Si5N8:Eu2+. The use of such red to yellow phosphors was disclosed with a blue light emitting primary source together with one or more red and green phosphors. The objective of such a material was to improve the red color rendition R9 (adjust the color rendering to red-shift), as well as providing a light source with an improved overall color rendition Ra.
U.S. Pat. No. 6,680,569 to Mueller-Mach disclosed a light emitting device having a (supplemental) fluorescent material that receives primary light from a blue LED having a peak wavelength of 470 nm, the supplemental fluorescent material radiating light in the red spectral region of the visible light spectrum. The supplementary fluorescent material is used in conjunction with a main fluorescent material to increase the red color component of the composite output light, thus improving the white output light color rendering. In a first embodiment, the main fluorescent material is a Ce activated and Gd doped yttrium aluminum garnet (YAG), while the supplementary fluorescent material is produced by doping the YAG main fluorescent material with Pr. In a second embodiment, the supplementary fluorescent material is a Eu activated SrS phosphor. The red phosphor may be, for example, (SrBaCa)2Si5N8:Eu2+. The main fluorescent material (YAG phosphor) has the property of emitting yellow light in response to the primary light from the blue LED. The supplementary fluorescent material adds red light to the blue light from the blue LED and the yellow light from the main fluorescent material.
Disadvantages of the Prior Art Blue LED-YAG:Ce Phosphor White Light Illumination System
The blue LED-YAG:Ce phosphor white light illumination system of the prior art has disadvantages. One disadvantage is that this illumination system produces white light with color temperatures ranging from 6000 to 8000 K, which is comparable to sunlight, and a typical color rendering index (CRI) of about 70 to 75. These specifications are viewed as a disadvantage because in some instances white light illumination systems with a lower color temperature are preferred, such as between about 3000 and 4100 K, and in other cases a higher CRI is desired, such as above 90. Although the color temperature of this type of prior art system can be reduced by increasing the thickness of the phosphor, the overall efficiency of the system decreases with such an approach.
Another disadvantage of the blue LED-YAG:Ce phosphor white light illumination system of the prior art is that the output of the system may vary due to manufacturing inconsistencies of the LED. The LED color output (quantified by the spectral power distribution and the peak emission wavelength) varies with the bandgap of the LED active layer and with the power that is applied to the LED. During production of the LEDs, a certain percentage are manufactured with active layers whose actual bandgaps are either larger or smaller than the desired width. Thus, the color output of such LEDs deviates from desired parameters. Furthermore, even if the bandgap of a particular LED has the desired width, during operation of the white light illumination system frequently deviates from the desired value. This also causes the color output of the LED to deviate from desired parameters. Since the white light emitted from the illumination system contains a blue component from the LED, the characteristics of the light output from the illumination system may vary as the characteristics of the light output from the LED vary. A significant deviation from desired parameters may cause the illumination system to appear non-white; i.e., bluish if the LED output is more intense than desired, and yellowish if less intense.
LEDs that emit in the visible, such as the prior art LEDs having an InGaN active layer, suffer from the disadvantage that a variation in the In to Ga ratio during the deposition of the InGaN layer results in an active layer whose band gap width which may deviate from the desired thickness. Variations in the color output of the phosphor (the luminescent portion of the white light illumination system) do not depend as much on compositional variations of the phosphor as they do on compositional variations in the blue LED. Furthermore, the manufacture of the phosphor is less prone to compositional errors than is the manufacture of the LED. Another advantage of using excitation wavelengths around 400 nm from the radiation source is that an LED such as GaInAlN has its highest output intensity around this range.
Thus, what is needed in the art is a white light illumination system with a radiation source emitting substantially in the non-visible, phosphors whose color output is stable, and whose color mixing results in the desired color temperature and color rendering index.